It has been known to produce a semi-frozen food product by delivering liquid and gaseous ingredients to a freezing chamber that refrigerates and agitates the liquid and gaseous ingredients to form a semi-frozen product. Some prior apparatus operate the freezing chamber at atmospheric pressure. In order to achieve a higher percentage of gas in the final product, apparatus have also been made which operate the freezing chamber at above atmospheric pressure. However, some difficulties have been encountered in maintaining a uniform proportion of gas and liquid in the final product under the varying operating conditions encountered during start-up and operation of pressurized type freezing and dispensing apparatus.
In some prior apparatus for preparing and dispensing a semi-frozen food product containing liquid and gaseous ingredients such as disclosed in Knedlik U.S. Pat. No. 3,044,878, the liquid ingredient is pumped into the freezing chamber under the control of a liquid level sensor and the gas is supplied at a preselected super-atmospheric pressure to the freezing chamber to maintain a gaseous head in the freezing chamber above the liquid. However, in such freezing apparatus, the amount of gas that mixes with the liquid in the freezing chamber is not accurately controlled but is dependent on several variables including pressure, temperature, time and amount of mixing, at least some of which will inherently vary during normal operation. Moreover, the gas in the freezing chamber expands during withdrawal of the product and tends to cause sputtering at the dispensing outlet.
Other apparatus, for example as shown in U.S. Pat. No. 3,460,713, pre-mix a gas such as carbon dioxide with the beverage producing liquid in a carbonator, and feed the pre-mixed liquid and gas under pressure into the freezing chamber. However, the amount of gas which is mixed with the liquid in the carbonator is also dependent on a number of variables including not only pressure but also temperature and time. While the pressure can be readily controlled, temperature control would require the use of additional refrigeration apparatus to cool the carbonator and maintain a constant temperature therein. The time variable cannot be readily controlled but is dependent on the average rate at which the product is withdrawn from the freezing cylinder. Moreover, the use of a carbonator and further refrigeration apparatus to control the temperature of the carbonator, markedly increases the cost of the apparatus.
More accurate control of the proportion of gas and liquid in the frozen product is achieved by the use of volumetric type liquid and gas pumps to feed the liquid and gas into the freezing chamber, as disclosed in U.S. Pat. No. 2,896,421 of Rader. The apparatus disclosed in that patent is arranged for operation only when the liquid and gaseous ingredients are supplied at substantially atmospheric pressure to the gas and liquid pumps. However, some gases, such as carbon dioxide gas used in making frozen slush-type beverages, are supplied in high pressure containers and it is difficult to accurately maintain the pressure on the gas supplied from such high pressured cylinders to the volumetric gas pump at substantially atmospheric pressure. Further, the volumes of gas that are utilized are frequently several times, for example from two to four or more times the volume of the liquid and, if the gas is supplied at atmospheric pressure to the gas pump, the gas pump would have to have a very large volumetric displacement as compared to that of the liquid pump.
It has also been proposed as disclosed in U.S. Pat. No. 3,591,051 to use a proportioning control in which a slide forms separate gas and liquid chambers in a housing and the slide is reciprocated to alternately expand and contract the volumes of the gas and liquid chambers in response to the pressures on the gaseous and liquid ingredients and under the control of solenoid valves operated in response to reciprocation of the slide. The pressures on the gas and liquid ingredients required to reciprocate the slide are substantially above the pressure to be maintained in the freezing chamber. High operating pressures aggravates the problem of providing and maintaining seals between the relatively moving parts. Further, the pressure of the water supply varies from time-to-time and in different localities and many water supplies cannot be relied upon to consistently provide water at pressures sufficiently above the pressure in the freezing chamber to enable operation of such a proportioning control.